The present invention relates generally to air-cooled internal combustion engines of the type utilizing rotational inlet screens to prevent the entry of foreign matter into the air intake ports. More particularly, the invention relates to such an engine wherein a tortuous entry path is formed for foreign matter entering the gap between the rotating inlet screen and the blower housing.
In an air-cooled combustion engine, air is the cooling substance. The barrier between the air and the hot gases in the engine is the engine cylinder and crankcase. These parts must present the greatest possible surface to the air if cooling is to be effective. An effective means of directing a stream of moving air over the surfaces must also be provided in the engine design.
These conditions are met in the cylinder blocks by providing cooling fins around the circumference, thereby presenting a greatly increased surface area to the air. Ventilation is necessary so that fresh air can constantly be blown across the engine to provide maximum cooling. Therefore, a fan or blower, which is incorporated in the flywheel, is utilized to direct a cooling stream of air across the cylinder block and other engine parts.
In normal operation, air-cooled engines are often exposed to air containing a large quantity of foreign matter, such as dirt, grass clippings, or matted debris. These materials enter the engine through the air intake port and collect on the surfaces of the engine and between the cylinder fins. The buildup of this foreign matter decreases the available surface area of the engine to the cooling air stream as well as insulates the hot metal from the air stream, often resulting in premature engine wear. Thus it is very important that all cooling surfaces be kept free from debris.
In order to reduce the amount of foreign matter that reaches the interior of the engine, screens are typically placed over the air intake port. These screens are either fixed or rotational. Generally more effective protection against the build up of foreign matter is provided by rotational screens since they are designed to rotate along with the engine cooling fan and fling foreign matter away from the screen as it nears the air intake port.
Although the use of rotating screens is generally effective in preventing the build up and entry of foreign matter through most of the air intake port, problems still exist with respect to foreign matter entering the engine through the gap defined by the peripheral edge of the rotational screen and the inlet edge of the air intake port of the blower housing.
In the past, the radial clearance between the rotational fan inlet screen and the blower housing has ranged from about 1/8 inch to 3/16 inch due to production tolerances of the components and the total tolerance stack up of the assembled components. Since precise gauging of the blower housing inlet and the rotational screen has not been practical, these relatively large clearance gaps between the inlet and the screen have allowed long grass blades, chaff, small leaves, and other foreign matter to enter the blower housing and be retained within the cooling fins of the cylinder and cylinder head, thus causing a build up of "bird nest" clusters, which restrict the flow of cooling air over the engine cooling fins and insulate the fins to prevent heat transfer from the engine.
In order to reduce the amount of foreign matter entering the interior of the engine through this clearance gap, it has been attempted to provide a tortuous path through which foreign matter must travel before entering the interior of the engine. Although such a path between the annular flange of the blower housing and the interacting peripheral skirt of the screen provides a longer path that foreign matter must travel before reaching the interior of the engine, there still exists the problem of buildup of foreign matter within the tortuous path. In addition, there still exists the problem of a relatively large clearance gap between the outer periphery of the rotational screen and the annular flange of the blower housing.